In recent years, hybrid vehicles and electric vehicles have attracted attention as energy saving and environmental conscious vehicles. Hybrid vehicles use motors as power sources in addition to conventional engines, and electric vehicles use motors as power sources.
Both the hybrid vehicle and the electric vehicle convert DC power stored in a battery into AC power by an inverter circuit and supply the power to the motor to run the vehicle.
Hereinafter, the configuration and operation of the control device of a conventional permanent magnet synchronous motor will be described with reference to the drawings. In the respective drawings, the same or corresponding parts will be described with the same reference numerals.
A detailed example of the conventional control device for a permanent magnet synchronous motor is shown in FIG. 9. As shown in FIG. 9, 7 denotes a DC power source, 6 an inverter, 301 a current detector, 4 a motor, 302 a magnetic pole position detector, and 309 an inverter control circuit.
Here, the control circuit 309 finally generates and outputs the gate pulse signals PU*, PV*, PW* for the respective phase switching elements of the inverter upon the input of the torque command value T*, hereinafter, the configuration will be described together with the operation.
In the rotary coordinate system rotating synchronously with the magnetic flux produced by the permanent magnet which is the rotor of the motor 4, the coordinate axis in the magnetic flux direction is defined as the d axis, the d-q axis coordinate system considers the coordinate axis in the direction perpendicular defined as the q axis. First, in the control circuit 309, 307 is a three-phase/two-phase converter that converts the phase current detection values IU, IW of the motor 4 by the current detector 301 using the magnetic pole position signal θ into DC current detection values Id, Iq that are components of the aforesaid d-q axis coordinate system.
On the other hand, 303 is a current command value generation section including a d-q axis current command calculation section for converting the torque command value T* into the d-q axis current command values Id*, Iq*, the d-q axis current command values Id* and Iq*, which are the outputs of the current command value generation section 303, input to the automatic weakening flux control calculating section 308. The calculation result of the automatic weakening flux control calculating section 308 is input to the current control system 304. The calculation of the automatic weakening flux control calculating section will be described later.
In the current control system 304, the deviation between the aforesaid d-q axis current Id, Iq calculated by the coordinate conversion section 307, and the d-q axis current command values Id*, Iq* are input. Further, the current control system 304 calculates the d-q axis voltage command values vd*, vq* by a proportional integral control using the aforesaid input deviation.
In the current control system 304, a non-interference control for canceling interference occurring between the d-q axes is performed.
The d-q axis voltage command values vd*, vq* calculated by the current control system 304 are input to the two-phase/three-phase converter 305 to calculate the three-phase voltage command values vU*, vV*, vW*.
The three-phase voltage command value calculated by the two-phase/three-phase converter 305 is input to the PWM modulator 306, generates the gate pulse signals PU*, PV*, PW*, and inputs them to the inverter 6.
In the case of driving a permanent magnet type synchronous motor, if the attempt is made to rotate at high speed, as the induced voltage becomes higher than the maximum voltage that the inverter can output, and the generator operates, the operation speed is limited. Therefore, a weakening flux control that weakens the magnetic flux by ostensibly allowing a negative d-axis current to flow and enables high-speed operation is used.
As a method of calculating the d-axis current for implementing the weakening flux control, a value that can be output by the power supply voltage is compared with the d-q axis voltage command value, and the deviation is set to 0 so that a method to control the d-q axis voltage command value by feedback control will be performed. In the following, a method of comparing the value that can be output by the power supply voltage with the d-q axis voltage command value and controlling the d-q axis voltage command value to a predetermined value by feedback control is called “automatic weakening flux control”.
In FIG. 9, 308 is an automatic weakening flux control calculation unit which compares a value that can be output by a power supply voltage with a d-q axis voltage command value, and a d-q axis current command value for causing the d-q axis voltage command value to follow a predetermined value by feedback control is calculated. The calculated current command value or the input current command value Id*, Iq* is selected based on a predetermined index, and input to the current control system 304.
As an index of the selection of the current command value, for example, the d-q axis voltage command value is compared with the voltage value that can be output by the power supply voltage (a voltage value in a range in which the induced voltage is not higher than the maximum voltage that the inverter can output), in the case where the d-q axis voltage command value is greater than or equal to the output voltage value, the command value calculated by the automatic weakening flux control is selected, otherwise, the input current command values Id*, Iq* are selected.
As a method of calculating the current command value, the center of the voltage limit ellipse (range of voltage to be limited within a range in which the induced voltage is not higher than the maximum voltage that the inverter can output) which can be output by the power supply voltage is calculated from the magnet magnetic flux and the inductance of the motor, there is a method of correcting the current command value in the center direction of the voltage limit ellipse (Patent Document 1).
However, with this method, when the motor temperature changes from the designed value, since the characteristics of the motor change, when the current command value is corrected, the combination of the d-q axis currents is not optimal with respect to the torque and the number of revolutions, there is a problem that the efficiency of the motor decreases and the efficiency of the combined motor and inverter decreases.
FIG. 10 is a diagram showing the task of the weakening flux control that corrects the current command value in the direction of the center of the voltage limit ellipse when the temperature of the motor lowers. In FIG. 10, 801 is a voltage limit ellipse calculated by the magnetic flux of the motor at the actual motor temperature, 802 is the center point of the voltage limit ellipse calculated by the magnetic flux of the motor at the aforesaid actual motor temperature, 803 is a voltage limit ellipse calculated by the magnetic flux of the motor at the motor temperature of the design value, and 804 is a center point of the voltage limit ellipse calculated by the magnet magnetic flux of the motor at the aforesaid designed motor temperature. The center point of the voltage limit ellipse on the d-q axis moves in the negative direction on the d axis when the magnetic flux of magnet of the motor increases, and moves in the positive direction on the d axis when it decreases. Further, in general, the magnetic flux of the magnet increases as the temperature decreases, and decreases as the temperature increases. Therefore, when the motor temperature decreases, the center point of the voltage limit ellipse moves in the negative direction on the d axis and when the motor temperature increases, the center point of the voltage limit ellipse moves in the positive direction on the d axis. In a conventional device, when the current vector 805 is input as the command value at the beginning, the current vector is corrected toward the point 804 by the automatic weakening flux control, stopped at the intersection point 807 with the voltage limit ellipse 801, and finally, the current vector becomes 806. On the other hand, the maximum system efficiency curve MXSEC determined by the torque and the number of revolutions of a certain motor is generally close to the maximum motor efficiency curve because the inverter efficiency is higher than the motor efficiency. Since the maximum motor efficiency curve MXSEC moves on the d axis in the same direction as the movement direction of the voltage limit ellipse, when the motor temperature decreases, the maximum system efficiency curve MXSEC moves in the negative d axis direction. Therefore, when the motor temperature decreases, since the deviation between the final current vector 806 and the maximum system efficiency curve MXSEC becomes large, the system efficiency decreases in the automatic weakening flux control of the Document 1.
Further, for example, a permanent magnet type synchronous motor, a winding type synchronous motor, as well as an induction machine can be used as the motor 4.